The invention relates to corrugated paperboard and shipping containers made therefrom.
In the preparation for market, transportation and storage of perishable comestibles such as vegetables, fruit and seafood, both the produce and the container are exposed to much water in one or all of the liquid, solid or vapor states. Such conditions have militated against the use, for that purpose, of containers made of paperboard.
In the marketing of fresh corn, lettuce, celery, peaches, and the like, the prevailing practice is to pack produce into its shipping container, in or near the field in which the produce is picked; and immediately thereafter to immerse the containers and contents in cold water to remove the field heat from them as quickly as possible. Thereafter, the packages are stacked, several high, in transportation vehicles or in cold storage chambers. The transportation vehicles are frequently refrigerated by ice, frequently in the form of flakes, charged into the body of the vehicle over and around the stacked packages of produce. Ice melts. In the course of the journey, depending upon its length and weather conditions, the shipment may be re-iced one or more times. Apples present a different aspect of the same problem. Apples are harvested in the autumn, but consumers expect to be able to buy them in other seasons. Apples are cold stored under high humidity conditions, e.g., 90% relative humidity, in boxes stacked ten to twenty feet high for months.
Ordinary corrugated paperboard containers lose their stiffness when wet. They collapse under the conditions described in the last paragraph. Damage to the packaged produce results. The higher the containers are stacked upon each other, the greater the load on the bottom one of the stack, and the greater the likelihood of collapse in the lower ones with resultant tendency of the stack to topple.
The problem is not a new one. It was referred to in U.S. Pat. No. 1,592,824 as follows:
"The use of boxes made of corrugated straw board has, up to the present time, been limited to uses where boxes and their contents are not subjected to moisture and becomes inefficient when wet or damp. For example, corrugated board boxes containing goods have not been kept in cold storage warehouses, because the moisture would cause the corrugated board to disintegrate under pressure of stresses and thus destroy the efficiency of the box as a container. A desideratum in this art has been to provide a corrugated board container which was made of faced corrugated straw board in usual manner and which would not be rendered incapable of performing its function when subjected to moisture." PA1 "The primary object of the present invention is to produce an improved corrugated board which has been treated, after the board has been manufactured, with a water-proofing compound, so that it will not absorb moisture." (Col. 1, LL. 10-34) PA1 "Certain chemicals can be used effectively to improve the compressive strength of paperboard at high humidities, with the phenolic resins being one of the more promising. The improvement is dependent upon the amount of the resin present in the board, the nature of the resin, and the way in which it is distributed in the board. Embrittlement of the board is one of the results of the treatment, but this is minimized with small levels of treatment. For the most efficient use of a specific amount of resin, surface treatments are more desirable than thorough impregnation of the board. Results point toward a skin effect on the board which may be achieved with treating mixtures of higher viscosity. Many chemicals that are not too effective at high moisture are highly effective at low moistures. PA1 "While the work to date shows some measure of success, further research is needed to achieve a board that will maintain its stiffness over the entire moisture range." PA1 "Compressive strength of linerboard exposed to high moisture conditions can be improved by treatments with water-soluble phenolic resins. The low-molecular-weight type resins gave the higher compressive strength, but these also cause more embrittlement of the board than the medium-condensed phenolic resins." PA1 "The use of phenolic resins for improving the wet compressive strength of paper is not new. However, one reason it has not been widely used in containers is that paperboard containing polymerized phenolic resin often becomes brittle. Obviously, a brittle corrugated fibreboard is difficult to score or fold without seriously fracturing the material at the score lines." PA1 "Roll suppliers generally offer a choice of three different cell shapes: the quadragravure, the pyramid, and the tri-helicoid. As noted, the quad is primarily designed for gravure type of coatings, with viscosities a bit heavier than water. The pyramid is for very aqueous formulations, since the sharp point of the upside-down pyramid retards release of the coating material from the cell. The tri-helicoid is primarily for highly viscous coatings like adhesives and asphalts."
It was further discussed in an article by D. J. Fahey entitled "Use Of Chemical Compounds To Improve The Stiffness Of Container Board At High Moisture Conditions," which appeared in Tappi, issue of September 1962, where data are tabulated concerning the physical properties of "Wet-Strength" paperboard sheets, of the kind used as components ("liner" and "medium") of corrugated container-board, to which various resins had been added at different stages (pulp slurry, size press, or smoothing press) of the paper-making process. Fahey concluded:
Comparable date on similarly treated paperboard are reported in U.S. Forest Service Research Note PPL-084, December 1964, with the following conclusion:
Another article entitled "Phenolic Resin Treatment Improves Fibreboard Compressive Strength" appeared in the October 1965, issue of Package Engineering, and reported:
It is the primary object of the present invention to provide corrugated paperboard, containers made therefrom, and a process of making the same, which, while providing rigid-when-wet strength sufficient to withstand the rigorous conditions of flooding with water, storage under high humidity, etc., while laden as related above, are not objectionably brittle.